Given the fact that this is pretty poor, I think i'll have to tender my own "Who, me?"
I was working for a pharmaceutical company and I had the pleasure of fitting an additional 512GB of memory to a test box. Unfortunately they didn't bother labeling up each server in the enclosure, I was given a map of what server was where. So I promptly shut the box down, pulled it out, fitted the memory and then booted it back up.
Zero issues, up until the following morning. I didn't bother using the UID on the machine to make sure it was right. Turns out I pulled the incorrect server, caused some application testing to fail and then fitted an astronmical amount of memory to it. The upside was the testing completed in a few hours with the extra memory instead of 24 hours, once that had finished, I shut it back down, shut the correct server down and moved everything.
I was cabling up an enclosure via fabric over to our san, Low and behold they never bothered to cut the cabling so by the time it got to the san, there was a massive pile of fiber, literally huge. So I started my work and traced the correct cable and plugged it in, all was fine, until I lost my footing on the flood and promptly put my foot into the pile of cabling. we lost 7 cables out of the hundreds that were there. So we had to re-cable them again.